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Section 10, Sail Shape
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Figure 10–1 Air Flow Around a Sail
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Figure 10–2 Angle of Attack
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Figure 10–3 Angle of Attack Held Constant
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Figure 10–4 Angle of Attack and Streamlines
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Figure 10–5 Air Flow, Sailing Directly Downwind
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Figure 10–6 Sail Shape, Draft
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Figure 10–7 High and Low Aspect Ratio Rigs
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Figure 10–8 Wind Variation with Height
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Figure 10–9 Wind Vanes
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Figure 10 – 10 Telltales
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Figure 10–11 How Telltales React to Sail Trim
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Vocabulary
Aspect Ratio. In general usage, the aspect ratio of a sail is taken to be the length of the luff to the length of the foot.
Attached Flow. The movement of particles along a surface, such as the flow of air particles along the
leeward side of a sail.
Backwinded. A sail is backwinded when another sail to leeward of it is trimmed so closely as to cause
increased pressure on the lee side of the windward sail thus causing the windward sail to bulge to windward.
Lift. A force generated on the leeward side of a sail by air passing over the airfoil shape. It is the force
that drives a sailboat. Also, when sailing close-hauled, a wind that shifts aft to allow a change of course toward the wind direction.
Separation. Detachment of air in a smooth laminar flow from the leeward side of a sail. See also “Stall.”
Stall. The condition that arises when the airfoil (sail) or hydrofoil (rudder) is turned too far off and the flow of air or water separates from the foil; there is no longer an attached flow.
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End of Section 10 Slides
